Add new system view, pg_config

Move and refactor the underlying code for the pg_config client
application to src/common in support of sharing it with a new
system information SRF called pg_config() which makes the same
information available via SQL. Additionally wrap the SRF with a
new system view, as called pg_config.

Patch by me with extensive input and review by Michael Paquier
and additional review by Alvaro Herrera.

src/include/common/config_info.h

@@ -0,0 +1,21 @@
+/*
+ * config_info.h
+ *		Common code for pg_config output
+ *
+ *	Copyright (c) 2016, PostgreSQL Global Development Group
+ *
+ *	src/include/common/config_info.h
+ */
+#ifndef COMMON_CONFIG_INFO_H
+#define COMMON_CONFIG_INFO_H
+
+typedef struct ConfigData
+{
+	char	   *name;
+	char	   *setting;
+} ConfigData;
+
+extern ConfigData *get_configdata(char *my_exec_path,
+								  size_t *configdata_len);
+
+#endif   /* COMMON_CONFIG_INFO_H */